The research is the first to observe the connection between plant-based estrogenic compounds, or phytoestrogens, and behavior in wild primates. In this case, it was a group of red colobus monkeys in Uganda.

As primates, we humans would likely experience similar effects from the compounds.

“It’s one of the first studies done in a natural setting providing evidence that plant chemicals can directly affect a wild primate’s physiology and behavior by acting on the endocrine system,” study lead author Michael Wasserman said.

He conducted the research as a graduate student at UC Berkeley’s Department of Environmental Science, Policy and Management.

“By altering hormone levels and social behaviors important to reproduction and health, plants may have played a large role in the evolution of primate -- including human -- biology in ways that have been underappreciated,” he added.

For 11 months, Wasserman and his team followed a group of red colobus monkeys in Uganda’s Kibale National Park and recorded what the primates ate.

For behavioral observations, the researchers focused on aggression, as marked by the number of chases and fights, the frequency of mating and time spent grooming.

The scientists also collected fecal samples to assess changes in hormone levels.

The researchers found that the more male red colobus monkeys dined on the leaves of Millettia dura, a tropical tree containing estrogen-like compounds, the higher their levels of estradiol and cortisol.

They also found that with the altered hormone levels came more acts of aggression and sex, and less time spent grooming -- an important behavior for social bonding in primates.

The tropical tree is a close relative of soy, which is also considered to be high in phytoestrogens.

Women going through menopause often take soy-based products to relieve some symptoms.